Open the picture in the Gimp, right click on the image, and select:
image - scale image
In the Pixel Dimensions section there is a drop down that says "px"
Change it to "in".
resize to whatever you want, select ok.
Right click on the pic and choose save or save as.
Alternately, if you want to print straight from the Gimp, right click
on the pic, choose print, there will be a slider to adjust the size
for printing.
If you think you will be using the Gimp from time to time, get this:
http://gimp-savvy.com/BOOK/
There is a link there to download an html tarball of the book.